1. Field of the Invention
The present invention relates, in general, to a ghost cancelling system, and more particularly to a ghost cancelling system that cancels ghosts from video signals by using a transversal filter.
2. Description of the Related Art
Usually, in television image receivers, a ghost cancelling system is provided for cancelling ghosts from the received signal. This ghost cancelling system is usually designed to cancel ghosts using a transversal filter (hereafter, TF).
As an example, this type of ghost cancelling system is described in Japanese Patent Disclosure (Kokai) No. 59-211315. The conventional technique discloses that the system is designed as a feed-back construction ghost cancelling system. Specifically, the video signal inputted from the input terminal of the ghost cancelling system is supplied to a subtractor circuit after being converted to a digital signal by an A/D conversion circuit. The subtractor circuit outputs a video signal from which ghosts have been cancelled by subtracting from the output of the A/D conversion circuit a ghost cancelling signal outputted from the TF. The TF generates the ghost cancelling signal using the output of the subtractor circuit. The output of the subtractor circuit which is supplied to the TF is successively delayed every cycle T (e.g., 70 nsec) by the tapped delay line. Each delayed output is added in an adder circuit after the tap coefficients have been supplied by the coefficient circuit connected to the corresponding tap. This addition output is supplied to the subtractor circuit as the ghost cancelling signal. Furthermore, these tap coefficients are corrected every vertical synchronising cycle, based on a reference signal.
Accordingly, although a video signal containing a ghost is obtained from the subtractor circuit in the commencement stage of the waveform equalizing process (ghost cancelling process), a ghost-cancelled video signal can be obtained in the final stage of the waveform equalizing process. In general, the correction of the tap coefficients is performed by using a microprocessor in a conventional manner. For example, the detail techniques of correction are described in "Ghost Clean System", IEEE Trans. on CE. vol. CE-29, No. 3, Aug. 1983, by Murakami, Iga and Takehara and "Automatic Ghost Equalizer with Digital Processing", IEEE International Symposium on Circuits and Stems, 1983, by Shimbo, Yasumoto, Miyata and Shiotani.
In the ghost cancelling system described above, however, when the reference signal contains a noise signal, a gentle waveform change is more difficult to detect than a steep waveform change. Thus, the gentle waveform change will not be sufficiently cancelled in the tap coefficient correction. As a result, the undulations of the gentle waveform appear in the output signal.